The present invention relates generally to the bending of glass sheets and, more particularly, to an improved apparatus for producing bent, tempered glass sheets without the use of any glass contacting bending fixtures.
Conventionally, the commercial production of bent, tempered sheets of glass, such as are used as glazing closures in automobiles and the like, include the steps of heating flat glass sheets to a temperature corresponding substantially to the softening point of the glass, bending the flat sheets into the desired curvatures, and then tempering the bent sheets by rapidly chilling the same to a temperature below the annealing range of the glass. The actual bending of the sheet is accomplished by permittng the heat-softened sheet to sag under the influence of gravity against a concavely curved shaping surface formed on an outline or skeleton-type bending mold or by pressing the heat-softened sheet between a pair of opposed complemental shaping surfaces. The subsequent tempering is usually effected by flushing the opposite surfaces of the heated sheet with a suitable cooling medium, such as air or the like, to chill the glass sheet and impart the desired stresses therein.
Still another technique for bending glass sheets in a mass production operation is disclosed in U.S. Pat. No. 3,396,000, assigned to the same assignee as the present invention, wherein the opposite surfaces of a heated glass sheet are differentially cooled as the sheet advances through the opposed blastheads of a chilling station in such a manner as to warp or bow the sheet to a preselected curvature without the use of a bending mold or any other bending fixture. This is done by varying the pressure or volume of air against the opposite surfaces so that one surface cools at a faster rate than the other surface. Traditionally, the upper surface is cooled at a faster rate than the lower surface to cause the sheet to arch or bend upwardly away from the conveyor rolls.
It is sometimes desirable to arch the glass downwardly such as for example when processing heated backlights, i.e., glazing closures having a printed heating circuit superimposed on the concave side of the glass or the inboard surface thereof as installed in an automobile. The fired circuit, generally formed of an electrically conducting silver-glass frit, must be located upwardly out of contact with the conveyor rolls while conveyed through the furnace to prevent the wetted or melted frit from smearing. Accordingly, the glass sheet must be bowed downwardly in a direction away from the frit circuitry. When processing such glazing closures, it is apparent that greater air pressure must be applied to the bottom surface than the upper surface of the glass sheet in order to cool such bottom surface at a faster rate to effect the downwardly directed curvature when the glass leaves the conveyor rolls. In order to prevent the glass sheet from fluttering or floating upwardly off the rolls into contact with the tubes of the upper blasthead as a result of this differential pressure, it has been found that a series of upper conveyor rolls in closely vertically spaced relation to the lower rolls can be employed to restrain upward movement of the sheet. However, problems are encountered in maintaining such rolls in a true horizontal, thereby inducing undesirable vibrations which adversely affect the desired curvature imparted to the sheet and often results in excessive glass breakage. Because of the closely spaced relation between the upper and lower rolls, allowing for only a small clearance between the glass sheet and the rolls, the size of the bearings for at least one opposite end of the roll must be compromised, rendering it incapable of adequately accommodating the loads imposed thereon. As a result, such bearings wear rapidly, causing the associated roll to wobble and run out during operation. Moreover, replacement of defective bearings and/or rolls poses problems because of limited access into the tempering section with consequent increased production downtime, adding materially to the costs of production.
Also, since all the tubes of the lower blasthead were formed integral with the upper plate of the manifold housing and arranged in a relatively closely spaced pattern across the entire surface area thereof, the removal of glass particles or cullet resulting from occasional glass rupture and breakage during the tempering operation posed problems. Moreover, damage to only a portion of the lower blasthead or only a few of the tubes required replacement of the entire lower blasthead.